1. Field of the Invention
This invention pertains to a novel mechanical system for controlling position of a rotating swashplate. More particularly, it relates to such a novel mechanical system which preferably also provides independent lateral cyclic, longitudinal cyclic and collective pitch control of a helicopter rotor attached to the swashplate. Most especially, it relates to such a novel mechanical system which provides direct, linear readout of cyclic and collective swashplate positions.
2. Description of the Prior Art
There are many swashplate control systems in use today, both in production and research helicopters and for use in wind tunnels and similar environments for evaluating helicopter rotor blade designs. Generally, the functions and movements that must take place in such systems are the same regardless of the particular mechanical design of the system.
The following background information on the operation of a helicopter is useful for understanding the present invention. A helicopter rotor generates lift by accelerating a mass of air downwards through its blades. The resulting lift is proportional to the mass and velocity of the downwash. In flight, the blades bend in an upward direction, called coning, until gravity, lift and centrifugal force balance. The helicopter engine supplies torque to overcome drag, which also acts on the blades. In forward flight, blades advancing toward the nose of the helicopter encounter faster apparent airflow, the vector sum of forward and rotational velocity, than blades moving toward the tail of the helicopter. As a result, advancing blades generate greater lift than retreating blades, creating an inbalance unless compensation is made.
To assure stablility, most helicopters use two forms of compensation, i.e., flapping and feathering. Flapping makes the blades respond to increased lift on the advancing side by rising to a maximum angle over the nose and falling to a minimum angle over the tail. The flapping changes the apparent angle that the blades attack the air, compensating for the airflow variations. Feathering causes the pitch of the rotor blades to be varied sinusoidally as the rotor spins to compensate for the sinusoidal airflow variations. The pilot uses both feathering and flapping to stabilize the helicopter.
To allow flapping and feathering, the rotor blades are usually hinged or flexible at their hub. Fully articulated rotors have hinges for lead/lag motions induced by drag variations in addition to flapping or feathering. Pitch variations are transmitted from the cockpit to the blades by means of the swashplate.
Besides stabilizing the helicopter, the cyclic pitch variations are also used to control it. The collective pitch, i.e., the average value of the cyclic pitch, determines the average lift of the rotor blade. The collective pitch is thus varied to make the helicopter climb, descend and hover.
Changing the amplitude and phase of the pitch cycle unbalances the rotor, causing it to tilt in a particular direction. The helicopter then accelerates in that direction until it regains a stable position at the new velocity. The pilot transmits control motions via two sticks. A collective stick pushes the swashplate up and down on the rotor mast, changing the collective pitch. A cyclic stick tilts the swashplate, changing the amplitude and phase of the cyclic pitch.
A single engine helicopter has a much better chance to make a safe landing following a power failure than a single engine airplane. The landing maneuver for the helicopter is not automatic (after power failure) and it requires some clever management of energy on the pilot's part to prevent damage to man or machine. Failure to make a good entry into autorotation after the engine stops is one of the primary causes of helicopter accidents. The key to making a good entry is to maintain rotor speed. The accepted way to stop rotor speed decay is to quickly reduce the power demands on the rotor by lowering the collective stick.
Existing swashplate control systems used in production helicopters usually incorporate a mixer box for cyclic and collective inputs prior to the swashplate. The mixer box is required to uncouple the collective, lateral cyclic and longitudinal cyclic inputs to the swashplate. This adds considerable complexity to the system.
Existing swashplate control systems used in wind tunnel test programs have uncoupled the cyclic and collective inputs by attaching the cyclic actuators to the collective slider assembly and allowing the actuators to float with the collective actuator inputs. Two cyclic actuators mounted 90 degrees apart about the swashplate rotation axis provide longitudinal and lateral cyclic inputs. However, because the cyclic actuators are both fixed to the same reference frame, when one of the cyclic inputs is large and the other cyclic input is increased, the first cyclic position changes by a small amount. Therefore, the two cyclic inputs are not truly uncoupled. With existing systems, direct, linear readout of the cyclic swashplate positions has not been possible.
Specific examples of prior art swashplate operating mechanisms are disclosed in the following issued U.S. patents. U.S. Pat. No. 2,978,038, issued Apr. 4, 1968 to Doman et al., discloses a swashplate operating mechanism with hydraulic servos for changing the angle of the swashplate longitudinally or laterally and for raising and lowering the swashplate. U.S. Pat. No. 4,235,116, issued Nov. 25, 1980 to Meijer et al., discloses a wobble plate drive mechanism with restraint provided by balanced gimbal rings. U.S. Pat. No. 4,445,421, issued May 1, 1984 to Walker et al. discloses a redundant swashplate control system which includes at least five actuators coupled to the swashplate. Despite the well developed status of the prior art, a need remains for further improvements in swashplate control systems to meet the needs of present and future helicopter development.